1. Field of the Invention
The present invention relates generally to transcoder/rate adaptor units within base station systems, and specifically to transcoder/rate adaptor unit protocols.
2. Background and Objects of the Present Invention
The Global System for Mobile (GSM) telecommunications network provides a set of network capabilities that are defined by standard protocols and functions that enable telecommunications services to be offered to millions of subscribers throughout the world. The GSM system has a modular network architecture with standardized circuit-switched interfaces between segments.
In a typical GSM system, the radio access network is commonly referred to as a Base Station System (BSS). Within the BSS are two logical nodes, the Base Station Controller (BSC) and the Base Transceiver Station (BTS). The BTS is responsible for radio transmission and reception in a single cell, and provides the capacity for handling a number of calls within the cell. Since GSM systems utilize the Time Division Multiple Access (TDMA) concept, each carrier frequency is divided into eight timeslots, which can be assigned with either control channels or traffic channels. Control channels carry information used by the network for supervision and management of calls, while traffic channels carry voice and data between subscribers involved in call connections. The traffic channels can be either full-rate, which occupies an entire timeslot or half-rate, which occupies only one-half of a timeslot.
The BSC interworks with a Mobile Switching Center (MSC) via an open interface, terms the A-interface, and one or more BTS's via respective Abis interfaces. One of the main functional responsibilities of the BSC is speech coding and rate adaptation. This responsibility is handled by one of a pool of Transcoder/Rate Adaptor Units (TRAUs) within a Transcoder Controller (TRC) in the BSC. Each TRAU is capable of transcoding Pulse Code Modulated (PCM) speech from 564 kilobits per second (kbps) on the MSC side to 16 kbps (for full-rate traffic channels with 13 kbps for speech and 3 kbps for signaling) on the BTS side. Thus, each TRAU can submultiplex four full-rate traffic channels on the BTS side into one 64 kbps PCM channel on the MSC side. For data, the TRAU adapts the rate between 8 kbps and 16 kbps.
In a typical call connection, the MSC requests assignment of a traffic channel via the A-interface to the BSC. In response, the BSC requests resources from the TRC via an Ater interface, which allocates a free TRAU, chooses a 64 kbps path towards a 64 kbps circuit at the A-interface and sets up a free 16 kbps circuit for a path towards the BSC. Thereafter, the BSC sets up a 16 kbps path to the BTS via the Abis interface. Finally, the BTS allocates the appropriate radio resources (timeslot) and a Channel Codec Unit (CCU), which is responsible for channel coding/decoding over the air interface.
One of the drawbacks of conventional circuit-switched GSM BSS's is the low transmission efficiency and complex connection switching involved in establishing call connections. Another drawback is that both the BSC and the TRC must handle both signaling and payload (speech or data).
To overcome these drawbacks, packet switching can be introduced into the BSS transmission infrastructure. In a packet based network, all types of traffic (signaling, speech or data) can be mixed together, provided that Quality of Service (QoS) standards are met. The packet switched BSS is based on the Internet Protocol (IP), which is the main stream packet based connectionless protocol.
In the packet switched BSS, the BSC is divided into a Radio Network Server (RNS) node, which handles only signaling, and a Media Gateway (MGW) node, which handles only payload and corresponds to the TRC. In this architecture, the payload is routed directly between the MGW and the BTS without passing through the RNS, whereas the signaling is routed between the RNS and BTS without passing through the MGW. For a complete description of the packet switched BSS architecture, reference is made to co-pending application Ser. No. 09/494,606, which is hereby incorporated by reference.
When changing the transmission structure in a GSM radio access network from circuit switched to a packet switched IP network, the protocols used between the various functional components may need to be modified to account for the new structure of the packet switched network. For example, the TRAU in the BSC and the CCU in the BTS communicate today through a protocol specified in GSM TS 08.60 and 08.61. This protocol is tightly coupled to the transmission structure of the circuit switched 64 kbps timeslots and sub-channels of 16 and 8 kbps. Therefore, when the transmission structure changes to a packet switched IP network, the TRAU protocol will not function properly.
It is, therefore, an object of the present invention to modify the TRAU protocol to operate within a packet switched IP based network.
It is a further object of the present invention to enable a connection between the CCU and the TRAU over an IP network in a simple and efficient way.